The present invention relates to a securing device for blades for gas turbines.
In particular, the present invention relates to a securing device for cooled blades for gas turbines, of the type used in the first stages of the turbine, which are the hottest stages, and a securing device for non-cooled blades, such as those used for subsequent stages of the turbines, which are the coldest stages.
The present invention also relates to plates for securing first- and second-stage blades for gas turbines.
As is known, gas turbines are machines which consist of a compressor and of a turbine with one or more stages, wherein these components are connected to one another by a rotary shaft, and wherein a combustion chamber is provided between the compressor and the turbine.
Subsequently, via corresponding ducts, the high-temperature, high-pressure gas reaches the various stages of the turbine, which transforms the enthalpy of the gas into mechanical energy which is available to a user.
In two-stage turbines, the gas is processed in the first stage of the turbine in temperature and pressure conditions which are quite high, and undergoes initial expansion in it; whereas in the second stage of the turbine it undergoes a second expansion, in temperature and pressure conditions which are lower than in the previous case.
It is also known that in order to obtain the maximum output from a specific gas turbine, the temperature of the gas needs to be as high as possible; however, the maximum temperature values which can be obtained in use of the turbine are limited by the resistance of the materials used.
Thus, owing to the high temperatures to which they are subjected, the blades which are used in the first stage of the turbines must be cooled, and for this purpose they have a surface which is suitably provided with holes for cooling of the outer surface of the ducts which permit circulation of air inside the blade itself.
In addition, in the root or foot of the blade, there are generally provided one or more ducts in order to permit supply and circulation of cooling air obtained from the compressor.
Unlike in the case of the first-stage blades, since the second-stage blades operate with gas at lower temperatures, in general they do not have these aeration ducts in their foot.
However, in both cases, a problem which occurs particularly according to the known art is that of guaranteeing optimum securing of the blades to the rotor disc, in all operating conditions of the machine.
In fact, it is known that the system for securing the blades to the rotor disc represents a crucial aspect of the design of any rotor, taking into account the fact that the latter must withstand loads which are generated by the blades, without giving rise to breakages or other similar disadvantages.
In fact, during operation of the machine, it is known that the rotor blades are subjected to high levels of stress, both radially, and to a lesser extent, axially.
The radial stresses are caused by the high speed of rotation of the turbine, whereas the axial stresses are caused by the effect produced by the flow of gas on the profiled surfaces of the blades.
The same flow of gas transmits to the blades the circumferential component of the stress, which makes it possible to gather useful power at the motor shaft.
However, the system for securing the blades must have the smallest possible dimensions, such as to reduce to the smallest possible dimensions the assembly constituted by the rotor disc and the blades.
The object of the present invention is thus to provide a securing device for blades for gas turbines, which has a low cost, and consists of a reduced number of component parts.
The device according to the invention thus has a structure which is extremely simple and compact.
Another object of the invention consists of providing a securing device for blades for gas turbines, which permits inflow of the air necessary in order to cool the blades of the first stage of the gas turbine, without creating problems of losses of load.
Another object of the invention is to provide a securing device for blades for gas turbines which permits easy assembly and dismantling of the blades of the various stages of the turbine, as required.
Another object of the invention is to provide a securing device for blades for gas turbines which has a high level of reliability.
A further object of the invention is to provide a securing device for blades for gas turbines which permits optimum resistance to the axial stresses which act on the blades.
These and other objects are achieved by a securing device for blades for gas turbines, of the type used for the first stage of the turbine, characterised in that it comprises a plurality of plates, each of which is provided with at least one U-shaped projection, which can engage with a corresponding U-shaped groove present in the surface of the disc of the first stage of the turbine, such that each of the said plates is interposed between two adjacent blades in order to secure the latter axially, but nevertheless permits passage of the supply of cooling air.
According to a preferred embodiment of the present invention, each of the U-shaped grooves present in the surface of the disc of the first-stage of the turbine is located at an outer portion of the disc, contained between two adjacent blades.
According to another preferred embodiment of the present invention, each of the securing plates has its own U-shaped projection at its own central part, whereas, when it is in the securing position, it has a pair of ends, both of which are folded at 90xc2x0 relative to their own longitudinal axis.
According to a further preferred embodiment of the present invention, the securing device for blades for gas turbines, of the type used for the second stage of the turbine, comprises a plurality of plates, each of which is interposed between the end portion of the foot of a corresponding blade and the disc of the second stage of the gas turbine, and each of which is provided with ends in order to secure the said blade axially.
According to a further preferred embodiment of the present invention, when seen in cross-section, the securing plates have a curved profile, with the concave part facing the cavity of the disc.
According to a further preferred embodiment of the present invention, when seen in cross-section, the plates have a plurality of cambers, provided at several points along their own longitudinal development.
Further characteristics of the invention are defined in the claims attached to the present patent application.